1. Field of the Invention
The present invention relates to an exposure apparatus, and more particularly, to a projection exposure apparatus in which a pattern on a mask is projected onto a photosensitive substrate and exposed by moving the mask and the photosensitive substrate in a predetermined direction with respect to a projection optical system.
2. Discussion of the Related Art
FIG. 7 illustrates the construction of a conventional projection exposure apparatus. A pattern on a mask 110 is projected onto a glass plate 114 (photosensitive substrate) at equal magnification via a projection optical system 112. In FIG. 7, the direction of movement (scan) of the mask 110 and glass plate 114 is taken as the X axis, a direction perpendicular to the X-axis in the plane of the mask 110 is taken as the Y-axis, and a direction normal to the mask 110 (i. e., the direction of the optical axis of the projection optical system 112) is taken as the Z-axis. The projection optical system 112 is installed at the center of a C-shaped bridge 116 (fixed support). An illumination optical system 118 includes a light source, such as an ultra-high-pressure mercury lamp, and a fly-eye lens, etc., and is installed on one end of the bridge 116 to illuminate a predetermined portion of the mask 110 with uniform brightness.
The mask 110 and the glass plate 114 are held on a mask stage 120 and a plate stage 122, respectively, such that the mask 110 and the glass plate 114 are substantially parallel to the XY plane. Furthermore, mask stage 120 and plate stage 122 are installed on a carriage 124 as an integral unit. Two Y-direction micromotion actuators 126 and 128 are installed on the carriage 124 beneath the mask stage 120 to adjust the position of the mask stage 120 in the Y direction. An X-direction micromotion actuator 130 is installed on the carriage 124 at the end portion of the mask stage 120 on the side of the projection optical system 112 to adjust the position of the mask stage 120 in the X direction.
The plate stage 122 is constructed in such a way as to be movable in the Z direction and tiltable about the X-axis and the Y-axis in order to substantially match the exposed region on the plate 114 with the pattern imaging plane of the mask 110 formed through the projection optical system 112 during scanning exposure. In other words, the imaging condition is adjusted by moving the plate stage 122 in the Z direction and by adjusting inclination of the glass plate 114 (i.e., tilting the glass plate 114 about the X-axis and the Y-axis). By performing such adjustments, it is possible to make corrections for thickness irregularities, inclination, and deformation, etc., which exist in the glass plate 114.
The carriage 124 can slide in the X direction along guide members 132a and 132b. When the carriage 124 is moved in the X direction with respect to illuminating light emitted by the illumination system 118, the mask 110 and the glass plate 114 are synchronously scanned by the illumination light from the projection optical system 112. This way, the pattern on the mask 110 is successively transferred onto the glass plate 114. Thus, the entire pattern on the mask 110 is projected and exposed onto the glass plate 114 by one scanning operation.
Next, an alignment mechanism for aligning the mask 110 with the glass plate 114 in the abovementioned projection exposure apparatus will be described. Moving mirrors 136a, 136b, 138a, and 138b are fixed to bottom portions of the mask stage 120 and plate stage 122 in respective positions corresponding to the Y-direction micromotion actuators 126 and 128. The moving mirrors 136a and 136b are arranged to reflect laser beams originating from a differential type laser interferometer 140 fixed to the carriage 124. More specifically, a laser beam emitted by the laser interferometer 140 is split into two laser beams by a split optical system 144, and the resultant two laser beams are guided to the moving mirrors 136a and 136b. The laser beams reflected by the moving mirrors 136a and 136b return to the laser interferometer 140 through the split optical system 144. At the interferometer 140, the two light beams reflected by the moving mirrors 136a and 136b are coupled to produce interference. Based on the interference information, the relative positional deviation between the mask 110 and the glass plate 114 in the non-scanning direction (i.e., the Y direction) is detected at a position corresponding to Y-direction micromotion actuator 126.
The moving mirrors 138a and 138b are arranged to reflect laser beams originating from a differential type laser interferometer 142 fixed to the carriage 124. More specifically, a laser beam emitted by the laser interferometer 142 is split into two laser beams by a split optical system 146, and the resultant laser beams are guided to the moving mirrors 138a and 138b. The laser beams reflected by the moving mirrors 138a and 138b return to the laser interferometer 142 through the split optical system 146. At the interferometer 142, the two light beams reflected by the moving mirrors 138a and 138b are coupled to produce interference. Based on the interference information, the relative positional deviation between the mask 110 and the glass plate 114 in the non-scanning direction (i.e., the Y direction) is detected at a position corresponding to Y-direction micromotion actuator 128.
Thus, the relative positional deviation between the mask 110 and the glass plate 114 in the Y direction can be detected by the laser interferometer 140 and the laser interferometer 142 at two points 126, 128, which are separated by a predetermined distance in the X direction. Furthermore, the relative rotational deviation about the Z direction between the mask 110 and the glass plate 114 can be detected from the difference in the results detected at the laser interferometer 140 and laser interferometer 142. When such deviations are detected, the Y-direction micromotion actuators 126, 128 are driven to offset the deviations. Furthermore, since the laser interferometers 140 and 142 utilize laser beams from light sources fixed to the carriage 124, the relative positional deviation detected in the Y direction is unaffected by changes in the attitude of the carriage 124. For example, even when the carriage 124 is displaced in the Y direction due to fluctuations in the X direction movement of the carriage 124, the light sources for the laser interferometers 140 and 142 and the split optical systems 144 and 146 are also displaced together with the carriage 124. Accordingly, no positional deviations between the mask 110 and glass plate 114 are detected in the Y direction.
A reflex mirror 148 and a reflex mirror 150 are disposed on the end portions of the mask stage 120 and plate stage 122, respectively, on the negative X direction side in the positions corresponding to the X-direction micromotion actuator 130. The reflex mirrors 148 and 150 are arranged to reflect laser beams from laser interferometers 152 and 154, respectively. The laser interferometer 152 is a length measuring type interferometer, and emits a laser beam from a light source toward the reflex mirror 148 fixed to the mask stage 120 and toward a fixed mirror (not shown in the figures) fixed to the bridge 116. Furthermore, this interferometer 152 detects interference (synthesis) between the laser beam reflected by the reflex mirror 148 and the laser beam reflected by the fixed mirror, and determines the position of the mask 110 in the X direction on the basis of the interference.
The laser interferometer 154 is also a length measuring type interferometer, and emits a laser beam from a light source fixed to a fixed system, such as the bridge 116 or the projection optical system 112, toward the reflex mirror 150 fixed to the plate stage 122 and toward the abovementioned fixed mirror (not shown in the figures). Furthermore, the interferometer 154 detects interference between the laser beam reflected by the reflex mirror 150 and the laser beam reflected by the fixed mirror, and determine the position of the glass plate 114 in the X direction on the basis of the interference.
Furthermore, the relative positional deviation between the mask 110 and the glass plate 114 in the X direction is detected from the difference in the results detected at the laser interferometer 152 and laser interferometer 154. More specifically, the relative difference between the position of the mask 110 in the X direction measured by the laser interferometer 152 and the position of the glass plate 114 in the X direction measured by the laser interferometer 154 is determined. Here, since light sources used for laser interferometers 152 and 154 are fixed to the fixed system (bridge 116 or the projection optical system 112, etc.), changes in the attitude of the carriage 124 in the pitching direction (direction of rotation about the Y-axis), i.e., the relative positional deviation between the mask 110 and the glass plate 114 in the scanning direction (the X direction) including the pitching amount of the carriage 124, can be detected. The output of the laser interferometer 154 at the plate stage 122 side is fed back to a carriage driving controller (not shown in the figures) to control the speed of the carriage 124 relative to the projection optical system 112 so as to produce uniform exposure across the entire area of the glass plate 114 during scanning exposure.
A long reflex mirror 156 extending in the X direction is fixed to the upper surface of the carriage 124 to reflect the laser beam emitted by a laser interferometer 158. The laser interferometer 158 is a differential type interferometer which detects changes in the attitude of the carriage 124 in the rolling direction (the direction of rotation about the X-axis). In this interferometer system, a laser beam emitted by a light source fixed to the bridge 116 is split into two beams and is guided to two points on the reflex mirror 156, which are separated along the Z direction. The laser beams reflected by the reflex mirror 156 are coupled to yield interference at the interferometer 158. According to the interference, the amount of rotation of the carriage 124 about the X-axis, i.e., the rolling amount, is detected. The positional deviations of the mask 110 and the glass plate 114 relative to the fixed system in the Y direction is determined on the basis of the rolling amount detected by the interferometer 158. This deviation is corrected by driving the Y-direction micromotion actuators 126 and 128.
In the conventional projection exposure apparatus described above, the laser interferometers 140 and 142 and the split optical systems 144 and 146 for the interferometers are fixed to the carriage 124. Accordingly, if the carriage 124 is deformed due to poor straightness of the guide members 132a and 132b, etc., a relative displacement is generated between the split optical system 144 and split optical system 146. As a result, the measured values by the laser interferometers 140 and 142, i.e., the relative positional deviation between the mask 110 and the glass plate 114 in the Y direction, may contain errors.
Furthermore, since the laser interferometers 140 and 142 are installed on the carriage 124, it is necessary to apply a large driving force to drive the carriage 124. Moreover, since the long reflex mirror 156 is fixed to the carriage 124 and the weight of the carriage 124 includes the weight of the reflex mirror 156, the driving force to the carriage 124 needs to be increased even further. As a result, the size of the driving system becomes undesirably large. With such a large driving system, it is difficult to achieve high scanning precision (uniform speed control, etc.) for the carriage 124.